nan Harsh

Trace level ammonia sensing by SWCNTs (network/film) based resistive sensor using a simple approach in sensor development and design

Highly stable and sensitive detection of trace level (approximately 1 ppm) of NH3 gas at room temperature is demonstrated for single wall carbon nanotube (SWCNT)-based resistive sensor. The sensor device was comprised of two planer Au electrodes deposited on carbon nanotubes (CNTs) by thermal evaporation method followed by patterning with photolithography process. For the growth of high-quality SWCNTs, multiple catalysts were deposited by co-sputtering method, and the yielded CNTs were in the diameter range of 0.8 to 1.5 nm. The CNTs were characterized by various techniques including Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. As-grown SWCNTs-based resistive sensor showed an excellent variation in sensor response in the trace sensing range from 1 to 50 ppm of NH3 where the sensor response linearly increased with NH3 concentration. The sensing mechanism was attributed to the electron transfer to the SWCNTs as a result of NH3 oxidation on the nanotube surface. The sensor was found to have good sensitivity with a response time of a few minutes at room temperature. Sensor recovery posed a great problem at room temperature, and the fast and complete recovery was successfully achieved by using appropriate thermal treatment protocol.

Effect of oxygen plasma on field emission characteristics of single-wall carbon nanotubes grown by plasma enhanced chemical vapour deposition system

Field emission properties of single wall carbon nanotubes (SWCNTs) grown on iron catalyst film by plasma enhanced chemical vapour deposition system were studied in diode configuration. The results were analysed in the framework of Fowler-Nordheim theory. The grown SWCNTs were found to be excellent field emitters, having emission current density higher than 20 mA/cm2 at a turn-on field of 1.3 V/μm. The as grown SWCNTs were further treated with Oxygen (O2) plasma for 5 min and again field emission characteristics were measured. The O2 plasma treated SWCNTs have shown dramatic improvement in their field emission properties with emission current density of 111 mA/cm2 at a much lower turn on field of 0.8 V/μm. The as grown as well as plasma treated SWCNTs were also characterized by various techniques, such as scanning electron microscopy, high resolution transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy before and after O2 plasma treatment and the findings are be...

Study of J-E Curve with Hysteresis of Carbon Nanotubes Field Emitters

We observe hysteresis in J-E plot during field emission measurement of CNTs grown by LPCVD technique. CNTs are synthesized on Fe-coated Si substrate at 650°C. SEM and Raman study confirm that CNTs are successfully grown on Si substrate by LPCVD technique. In this study, we find that ramp-down curve has higher value of current density than ramp-up curve which indicates that CNTs show positive hysteresis. Our results show that a high current density at low turn-on voltage is obtained in ramp-down step of J-E plot which may be since not all CNTs contribute in ramp-up step process. But in ramp-down step all CNTs contribute as field emitters due to high electric field treatment. We also performed stability analysis of CNTs with current at constant applied voltage for 5 hrs and find that the sample shows long-term stability due to increase in emitting site density since a large number of CNTs participate in field emission.

A sensitive detection electronics for resistive sensor

Present work deals with the development of a low cost, appreciably accurate precision electronic circuit for resistive sensor where measurement of the incremental resistance with high degree of accuracy is essential. A linear and sensitive active bridge circuit requiring only few components for its hardware implementation has been proposed for measuring very small resistance change due to change in physical quantity or chemical analytes. Theory of the proposed active bridge circuit has been discussed and experimental results have been compared with conventional bridge circuit. Initial measurements are made with Pt-100 Strain gauge sensor but it can be extended to other resistive sensors of practical importance. Results show that the active bridge circuit is almost four times more sensitive than conventional full bridge circuit.

Effect of Growth Temperature on the Diameter Distribution and Yield of Carbon Nanotubes

Growth of carbon nanotubes (CNTs) on iron sputtered Si substrate has been done by using self design Thermal Chemical Vapor Deposition (TCVD) at atmospheric pressure. Parameters of CNTs are highly dependent on the growth temperature. A strong relation between CNT’s diameter, yield and growth temperature was found. The experiments were done in the temperature range of 750–900 °C with an interval of 25 °C. It was found that at 750 °C there was no growth of CNT. However, at 775 °C, the horizontal network of CNTs having diameter range of 8–12 nm with sufficient yield was observed. As we increase the temperature, an increase in CNT’s diameter and decrease in yield was found. These results demonstrate that diameter and yields of CNTs can be controlled with the growth temperature.

Raman Characteristics of Vertically Aligned Single Wall Carbon Nanotubes Grown by Plasma Enhanced Chemical Vapor Deposition System

Vertically aligned single wall carbon nanotubes (VA-SWCNTs) of diameter 0.8–1.5 nm suitable for semiconducting applications have been successfully grown on Iron catalyst film using Plasma Enhanced Chemical Vapor Deposition (PECVD) System. The Raman signal positions of the spectra in RBM, D and G bands confirm the existence of SWCNTs. The grown sample is excited with laser excitation wavelengths, 633 nm from He–Ne laser. The field emission study has been carried out in a vacuum chamber under a pressure of 10−6 torr. Highly sensitive, capital intensive equipment such as Field Emission Scanning Electron Microscope (FESEM), has been used to identify the state and morphology of nanotube samples.

Enhancement of Field Emission Properties of Carbon Nanotubes by ECR-Plasma Treatment

We report a significant improvement in electron field emission property of carbon nanotubes film by using an electron cyclotron resonance plasma treatment. Our research results reveal that plasma treatment can modify the surface morphology and enhance the field emission characteristics of carbon nanotubes. Raman spectra indicate that plasma treated CNTs sample has lesser defects. Before plasma treatment, low current density of 6.5 mA/cm2 at 3.0 V/μm and at a high turn-on field of 2.4 V/μm was observed. ECR plasma treated CNTs showed a high current density of 20.0 mA/cm2 at 3.0 V/μm and at a low of 1.6 V/μm. The calculated enhancement factors are 694 and 8721 for ECR-plasma untreated and treated carbon nanotubes, respectively. We found an increase in the enhancement factor and emission current after the ECR-plasma treatment. This may be attributed to creation of geometrical features through the removal of amorphous carbon and catalyst particles.

Field-Emission Study of Carbon Nanotubes Grown by Low Pressure Chemical Vapour Deposition on Single and Dual Layer of Catalyst

In the present research work, CNTs are synthesized by Low Pressure Chemical Vapor Deposition (LPCVD) method at 600 °C. The Si substrate is coated with Ni (single layer) in one sample and Ni over Cr layer (dual layer) as a catalyst on the other sample by using RF- sputtering method. Three precursor gases Acetylene (C2H2), Ammonia (NH3) and Hydrogen (H2) with flow rates 10, 50 and 50 sccm respectively are allowed to flow through the tube reactor for 20 min. Acetylene is used as source gas and Ammonia to etch the amorphous carbon and for the further reduction of catalyst size. The as grown CNTs sample was characterized by Scanning Electron Microscope (SEM) and Raman. Raman Spectra show the graphitic nature of CNTs grown on dual layer of catalyst. Field enhancement factor is increased in the dual layer coated samples.

Synthesis of Graphene by Low Pressure Chemical Vapor Deposition (LPCVD) Method

Graphene , the wonder material, is a one atom thick two dimensional crystal lattice having a honeycomb like structure. Its peculiar electrical, mechanical and optical properties have attracted the attention of the researchers like never before. Graphene film with two dimensional structure were successfully prepared via a physical method on Ni coated Si wafer using low pressure chemical vapor (LPCVD) method at temperature as low as 650 °C. For this growth acetylene was used as source gas and the hydrogen as the carrier gas in ratio of 1:20. The as-grown graphene was characterized using Scanning Electron Microscopy, Fourier transformation infra-red (FTIR) and Raman Spectroscopy. The SEM, FTIR and Raman spectroscopy confirmed the successful growth of multilayer graphene.

Highly Efficient Field Emission Characteristics of Ultra-long Vertical Aligned Single Wall Carbon Nanotubes

In this work, ultra long vertically aligned single wall carbon nanotubes are synthesised by Plasma enhanced chemical vapour deposition (PECVD) technique at 600 °C temperature. The presence of built-in electric field in a plasma sheath aligns the growing CNTs along the field lines. Also, PECVD method favours low temperature synthesis of VA-SWCNTs. SEM and Raman are used to characterized as grown sample. Enhanced Field emission properties of as-grown VA-SWCNTs are also studied.